Forced circulation evaporators are widely used in industry, primarily for handling the more difficult evaporation duties. Examples are crystallizing services, where positive circulation is needed to keep the crystals in suspension; handling fouling liquids, which must be circulated past the heating surface at high velocity to prevent deposition of foulants; handling viscous liquids, which exhibit very poor heat transfer characteristics unless circulation past the heating surface is forced; and handling highly corrosive liquids, where the better heat transfer attainable with forced circulation permits a substantial reduction in heat transfer area of expensive alloys. Probably the earliest forced circulation evaporator was the propeller calandria, in which short vertical heat transfer tubes were arranged in circular tubesheets having a large diameter central downtake tube. A propeller rotating in this tube recirculated liquid down through the downtake and up through the heat transfer tubes to increase tube velocities and hence heat transfer performance, and also to help keep crystals in suspension. This type evaporator is still in widespread use, primarily for crystallizing common salt from brines. Most present-day forced circulation evaporators follow the teachings of Sadtler (U.S. Pat. No. 1,246,939, Nov. 20, 1917) and employ a conventional external pump to recirculate the liquid past the heating surface. The most common modern version is shown in FIG. 1. Liquid to be concentrated is circulated by the pump through a shell-and-tube heat exchanger, where it is superheated slightly (usually about 5.degree. to 15.degree. F.). The liquid is then discharged to a flash chamber or vapor head where vapor is flashed off and the remaining liquid, plus makeup feed, is returned to the pump for recirculation to the heater. This is the general operating cycle I employ except that I combine the duties of pump and vapor head in a single device.
Another version of forced circulation evaporator that bears some resemblance to mine is the wiped film evaporator first disclosed by D'Yarmett U.S. Pat. No. 1,870,445, Aug. 9, 1932). In this type, an impeller is used both to circulate liquid past the heating surface and to effect vapor-liquid separation, as does mine. However, the heating surface in these evaporators is the shell housing the impeller. The heating surface is then simply a large diameter cylindrical or tapered tube and it is impossible to provide substantial amounts of heating surface area in a reasonable size evaporator. Further, since the tube is heated from the outside, usually by steam, it is subjected to a collapsing pressure that necessitates use of a thick wall for the large diameter tube. The thermal resistance of this wall makes it most difficult to achieve high heat transfer performance.